Promote libffi-dev as a general dependency

libffi-dev/libffi-devel can be requested for rebuilding the
python clients which is an un-skipable part of the devstack install.

Promoting the libffi-devel to a general dependency.

Change-Id: I1dac7fd536b94de1e49aaf335ac29ffae1bd39d1
diff --git a/files/apts/general b/files/apts/general
index 90529e5..4cd9303 100644
--- a/files/apts/general
+++ b/files/apts/general
@@ -23,3 +23,4 @@
 python2.7
 bc
 libyaml-dev
+libffi-dev
diff --git a/files/apts/glance b/files/apts/glance
index b5d8c77..15e09aa 100644
--- a/files/apts/glance
+++ b/files/apts/glance
@@ -1,4 +1,3 @@
-libffi-dev
 libmysqlclient-dev  # testonly
 libpq-dev           # testonly
 libssl-dev          # testonly
diff --git a/files/apts/swift b/files/apts/swift
index 080ecdb..fd51699 100644
--- a/files/apts/swift
+++ b/files/apts/swift
@@ -1,5 +1,4 @@
 curl
-libffi-dev
 memcached
 python-configobj
 python-coverage